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Seismic attenuation parameters in Taiwan have been obtained by using a modified "MultipleLapse Time Window Analysis" approach in order to take into account the scatteringanisotropy. MLTWA method exploits the spatio-temporal dependence of the ratio betweencoherent and incoherent seismic intensity to determine the scattering and absorptionproperties of a heterogeneous medium. In addition to the mean free path l (or the scatteringquality factor Qsc), the seismic wave propagation is controlled by a parameter g(independent of l) which determines the angular redistribution of energy upon scattering(scattering anisotropy). Preferentially forward (resp. backward) scattering correspondsto g > 0 (resp. g < 0), with g = 0 for isotropic scattering. We use Monte-Carlosimulations to compute Green functions and sensitivity kernels for Qsc, Qi (absorptionquality factor) and g in media with an heterogeneous crust overlying a transparentmantle. We analyse seismic records of local earthquakes with magnitude greater than 3.5which occurred between 1994 and 2016. For each stations, we collect events withina radius of 100 km and a maximum depth of 40 km. About 350000 waveformswith a S/N ratio greater than 4 in the coda are selected. Seismic energy in three15 s time windows from the S-wave arrival are measured in 3 frequency bands(1-2, 2-4,4-8 Hz). To account for site amplification and source amplitude, wenormalize observed energy by the average energy in a late coda window at 65 s fromthe origin time of the earthquakes. For every station, we analyse the normalizedenergy in the three time widows as a function of the hypocentral distance. In order todetermine the attenuation parameters Qsc−1, Qi−1 and g with related errors, weimplement a Levenberg-Marquardt algorithm. To guide the inversion, we use thesensitivity kernels for the attenuation parameters computed with our Monte-Carloapproach. Overall, Taiwan is more attenuating than most orogens with a mean effective scatteringloss (Qsc∗)−1 = Qsc−1(1 − g) about 0.012 and a mean intrinsic absorption Qi−1 about0.009 at 1.5 Hz. Our maps reveals strong lateral variations of attenuation with scale lengthfrom 10 to 100 km. Scattering loss (Qsc∗)−1 varies over more than one order of magnitudeacross Taiwan (between 0.0022 and 0.16 at 1.5 Hz). By contrast, absorption fluctuations areabout 30%. The more attenuating zones are the Coastal Range and the Costal Plain wherescattering dominates over absorption at low frequency, and inversely at high frequency. Theseregions are also characterized by strong backscattering (g < −0.85) at 1.5 Hz and ratherhigh V P∕V S ratio. Scattering becomes much more isotropic at high frequency.We speculate that the observed strong backscattering at low frequency is relatedto strong impedance fluctuations in the crust induced by the presence of fluids. [ABSTRACT FROM AUTHOR]

The lapse-time dependence of the quality factor Qc measured from the S-wave codaof local earthquakes at frequencies above 1 Hz has long been proven effective toinvestigate separately the scattering and absorption properties of the crust. On the otherhand, recent studies demonstrate that the Green’s function retrieved from the noisecross-correlation (NCF) between two stations contains a tail of multiply-scattered waves afterballistic waves like the coda of earthquakes, the decay rate of which can thereforebe also exploited for mapping of subsurface attenuation structures. In this study,we estimate frequency-dependent Qc of the coda in the dominant period range ofsecondary microseisms (3-9 s) derived from the yearly-stacked NCFs between allthe available station pairs separated by less than 80 km apart in Taiwan, which isknown as one of the most seismically hazardous and geologically complex regions inthe world. To ensure the Qc measured by the coda at sufficiently long lapse timeto be representative of the physically meaningful intrinsic quality factor (Qi) assuggested by Calvet and Margerin (2013), we choose different onset times (tw)of the coda at 10 or 20 s after the direct surface wave arrivals calculated by theinter-station distance divided by the average group velocity and measure Qc as afunction of the length of the coda window (Lw). The Qc values at periods of 3-6 sand 4.5-9 s determined with different choices of tw and the fixed Lw reveal noobvious trend with the inter-station distance. However, similar to the Qc observationsfrom the earthquake coda, they generally follow a transient increase with Lw atrelatively short lapse times and then approach a stabilized plateau value at Lw ∼75-100 s considered as an optimal approximation of Qi. We preliminarily map thelateral variations of intrinsic attenuation by simply distributing the determined Qcuniformly along the corresponding inter-station path and taking the mean values ofQc within individual 0.2∘× 0.2∘ cells. The results show that the Qc variation atthe shorter periods of 3-6 s is more pronounced and the lowest (∼50), implyingthe highest intrinsic attenuation, in the Coastal Plain of SW Taiwan covered withthick sedimentary deposits. It gradually increases toward the Western Foothillsfold and thrust belt in south central Taiwan and reaches the highest (∼100) in theHsuehshan Range in north central Taiwan, which seems to be correlated with theprogressive increase of the metamorphic grade in the Taiwan orogen as a result of thesouthward-propagating arc-continent collision. Further attenuation tomographyusing the proper sensitivity kernels will be conducted to reveal both lateral anddepth variations of the absorption properties across diverse litho-geological units inTaiwan. [ABSTRACT FROM AUTHOR]